Duncan Sparrell and I met with the MQTT TC chair and one of the spec editors for MQTT v5 to discuss our transfer specification, and they offered some useful insights. One that I think warrants discussion regards message sequencing & potential
duplication. Since we're recommending QoS 1 for our purposes, the info in MQTT v5 sections
4.3.2 (QoS 1),
4.6 (Message Ordering) and
4.9 (Flow Control) is relevant. Basically, with QoS 1, at least once delivery of every message is assured and "the final copy of each message received by the subscribers will be in the order that they were published", but there's the potential for duplicate
messages (section 4.6 explains this well). The "Receive Maximum" field in a CONNECT packet can be set to 1 to ensure messages at QoS 1 arrive strictly in order but it's not clear to me that we want to limit message flow in that way. Right now our specification
makes no recommendations regarding "Receive Maximum" (which defaults to 64K messages). Any thoughts regarding flow control and message duplication are welcome.
Dave
David Lemire
Systems Engineer
HII Mission Driven Innovated Solutions (HII-MDIS)
Technical Solutions Division
302 Sentinel Drive | Annapolis Junction, MD 20701
Work (301) 575-5190 |
Mobile (443) 535-1182